Horn-loaded loudspeaker



March 11, 1969 B, COHEN 3,432,002

HORN-LOADED LOUDSPEAKER Filed May 1 1967 PRIOR ART j r Z 5 pi A A! j g 5 1 r AR m r I f \Pa A 00 200 500 I000 5000 20000 FREQUENCY IN CYCLES PER SECOND FIG 2 FIG 4 ABRAHAM B. COHEN.

INVENTOR BY M 20 AGENT V United States Patent 1Claim ABSTRACT OF THE DISCLOSURE A loudspeaker is disclosed herein which includes a three-bend folded horn, a conventional compression driver, and a protective housing to enclose the compression driver and isolate it from environmental gases so that the loudspeaker is characterized as being explosion proof.

This invention relates to loudspeakers and more particularly to a loudspeaker having a horn in combination with a driver unit.

Reflex or folded-horn loudspeakers have in the past provided good results for certain applications, namely, where somewhat limited high frequency response is at least tolerable and in some cases even desirable. This is because the reflections and differences in length of sound paths within the bends of the horn cause cancellation of some frequencies, particularly in the high frequency audible range. Because of the problems surrounding folds in horns, the number of such folds has usually been kept to a minimum, with most reflex loudspeakers having only one or two folds.

There are techniques, however, for at least partially solving the problem of phase inequality which attends the bending of the air or sound column in a reflex loudspeaker. For example, the patent to Sidney E. Levy, 2,746,558 describes one of the more successful folded-horn constructions, the efficiency of which is primarily dependent on the proper proportioning of the height and width dimensions of the sound column throughout its length. It may be noted, however, that even with the increased efiiciency based on the geometry of the Levy sound column, a two-bend horn comprises the preferred embodiment.

An inherent characteristics of the popular two-fold loudspeakers is that since the output of the horn is always directed to the front, and since there are two bends in the sound path, the sound waves as they originally emanate from the driver unit must have a forward orientation. This is accomplished by mounting the driver unit at the rear of the horn and having the sound output aperture oriented for emitting sounds in a frontal direction. Such a construction does have some limitations, however, particularly when a driver unit must be replaced; for in such a case, access to the rear of the loudspeaker must be possible. If there is insuflicient room for a person to gain access to the rear of the loudspeaker, either the horn and the attached driver unit must be rotated from their normal operating position to a servicing position, or the entire assembly must be removed from its mounting location. While such movement of the horn may be of little consequence in many instances, there are cases in which this would not be desirable. For example, it is known that the energy of sound waves propagated by a horn-loaded loudspeaker is a highly directional quantity; and, having once discovered or calculated a favorable orientation for the born to direct this energy as required, it would be preferably not to disturb the horn merely to replace the driver unit. Too, the housing of the loudspeaker or the rim of the horn may be permanently mounted in such a way that removal of the entire loudspeaker is impractical or at least undesirable.

3,432,002 Patented Mar. 11, 1969 While it has been known to make horns such that the driver unit can be reached from the front of the loudspeaker, such speakers have usually been noted for their long or their bulky shapes. These loudspeakers in the prior art are thus not as satisfactory as is desirable when space is at a premium, or when weight is an important consideration in selection.

The liabilities of weight and bulk take on even greater significance when a requirement of making the loudspeaker explosion-proof is added.

The typical manner of making a loudspeaker explosionproof is to enclose the driver unit (and all electrical contacts) in a relatively heavy protective housing such that any explosion that might possibly occur at the driver unit will be contained and not allowed to set off a larger explosion in the ambient environment. Such a protective housing, when it serves its intended purpose, is a major asset. In another aspect, however, a protective housing as employed in the prior art is a burden; that is, it is dead weight and has no other function except to guard against the spread of an explosion that everyone hopes will never occur in the first place.

Accordingly, it is a major object of this invention to provide an explosion-proof loudspeaker having a compact design.

Another object is to provide a loudspeaker design which permits the fabrication of relatively light-weight, explosion-proof loudspeakers.

A further object is to provide a reflex loudspeaker in which the driver unit is removable from the front of the loudspeaker.

Yet another object is to provide a three-bend loudspeaker which is very compact and which presents a relatively thin silhouette.

A still further object is to provide a three-bend loudspeaker which can be assembled from a small number of component parts.

Other objects and advantages will be apparent from the specification and claims and from the accompanying drawing illustrative of the invention.

In the drawing:

FIGURE 1 is a partially sectioned, elevation view of a compression-driven, horn-loaded loudspeaker having a folded horn which is characteristic of the prior art;

FIGURE 2 is a partially sectioned, elevation view of one embodiment of a loudspeaker made in accordance with the invention;

FIGURE 3 is an isometric view of one of the filters shown in FIGURE 2; and

FIGURE 4 is a performance curve showing results obtained with several embodiments of the invention having different combinations of filters.

With initial reference to FIG. 1, a loudspeaker assembly 10 is shown which is typical of prior art loudspeakers that have been approved and rated as explosion proof. The driver unit 11 is protected by placing it within a relatively heavy and somewhat bulky housing 12 which isolates the driver from the ambient environment. Threaded means are employed to connect the driver unit 11 with a trumpet or horn 14 through which the sound is propagated. As can be seen from inspection, the driver unit 11 is mounted toward the rear of the assembly 10, and it can be disengaged from the horn 14 only by removing at least part of the housing 12 and subsequently moving the driver rearwardly with respect to the horn. The space which is required to permit removal of the driver unit 11 and its housing .12 dictates that the mounting bracket 15 be rather large, especially if replacement of the driver is to be accomplished without disturbing the position of the born 14.

With reference to FIG. 2, a substantially round louda speaker 16 which comprises one embodiment of the invention is shown partially in cross section in order to best compare it with the loudspeaker of FIG. 1. It should be understood, of course, that the size differential of the two figures is not intended to be significant; rather, as will be made clear herein, it is a difference in shape and, among other things, a novel relationship of parts that structurally distinguishes the two loudspeakers and contributes to different results.

As is common whenever it is desired to achieve an exponentially expanding air column which is as long as possible in a relatively small space, a folded horn 17 is employed. While the horn is represented as being round, other shapes such as rectangles, squares, etc., have been employed using the concepts disclosed herein, and it should be understood that it is not intended to limit the invention to round horns. The horn 17 comprises a plurality of annular elements having surfaces that reflect and bend the sound waves as they pass from a throat 18 to a mouth 19, with the mouth quite naturally defining the front of the loudspeaker 16. A driver unit 20 having an enveloping shell 35 with a sound output aperture 21 is affixed in some manner such as by threads in a position Where the sound output aperture is in communication with the horn throat 18. As shown, the sound output aperture '21 is arranged to propagate sound waves in a substantially rearward direction with respect to the loudspeaker 16. A first or inner element 22 and successive elements 23, 24 and 25 are adapted to guide the sound waves without causing undue distortion through three changes of direction, i.e., from the original substantially rearward direction to a concluding substantially forward direction. The sound channel, then, between the throat 18 and the mouth 19, comprises a serial array of annular segments bounded by the confronting surfaces of the elements or walls 22, 23, 24, 25.

The driver unit shell '35 envelops a magnet structure 36, a voice coil 37, and a vibratile element or diaphragm 38, such that an assembly is provided which is removable from the loudspeaker as a unit.

A cover member 26 having a smooth annular surface 27 is adapted to be affixed interiorly of the horn 17 where it cooperates with the inner element '22 to enclose all but the sound output aperture 21 of the driver unit 20. The annular surface 27 is adapted to bear against a confronting annular surface 28 on element 22, and, when held together by bolts or the like, the inner element 22 and the cover plate 26 comprise structure which isolates the driver unit 20 from the ambient environment; thus, it functions in the same manner as does the housing 12 in FIG. 1. The cover 26 is preferably installed in such a manner that it can be later removed in order that the driver unit 20 may be similarly removed, for repair, replacement, etc. In addition to enclosing the driver unit '20, the structure consisting of walls 22, 26 quite naturally also encloses a quantity of air; that is, of the matter contained in the space enclosed by the walls, everything that is not solid is gaseous. The quantity of air will vary, of course, from one embodiment to the next, to the extent of variations in the spacing and size of the wall 22, the cover plate 26, and the driver 20. While the presence of air around the driver unit 20 is of no initial consequence, there exists the definite possibility that an explosive gas will bleed between the surfaces 27, 28 into the enclosure. Thus, after an extended period of time, the gas within the walls 22, 26 must realistically be considered to be potentially explosive. In any event, the walls 22, 26 are made sufficiently strong in relation to the volume of gas enclosed thereby, so that, if the gas were somehow to be ignited and an explosion resulted, the walls will effectively resist rupture.

The ability of the walls 22, 26 to resist rupture from an internal explosion is not ideally the result of wall strength alone. In addition to wall strength an advantageous feature that contributes to explosion-proof properties is the construction of the interface between surfaces 27, 28 such that it is deliberately not made 100% air-tight. Hence, if an explosion does somehow occur within the walls 22, 26, a substantial portion of the hot gases resulting from the explosion will vent themselves through the microscopic channels between surfaces 27, 28 and thereby relieve the pressure tending to rupture the walls. The escaping, explosion gases themselves pose no problem because they are cooled to a safe value as they travel the relatively long paths before reaching the ambient gases outside of the walls 22, 26. Most persons will recognize, too, that if the volume of enclosed gas is, for example, decreased, the design strength of the walls 22, 26 can be relaxed by making them thinner or using a weaker material, etc. Conversely, enlarging the quantity of gas will generally increase the strength requirement in order to preclude rupture due to an explosion. It is believed, therefore, that no precise dimensions need be quoted in 'order to adequately describe the walls 22, 26, since those skilled in the art will understand what is meant when it is said that, in the illustrated embodiment, the structure surrounding the driver unit is explosion proof.

It is worthy of noting at this point that the Wall 22 serves a double purpose in that is serves as part of the enclosure for the driver unit 20 as well as part of the horn structure 17. By arranging the loudspeaker components as shown, such that a single wall 22 is common to both the horn 17 and the structure protecting the driver unit 20, at least one wall has been eliminated and a loudspeaker of fewer component parts is provided. A further advantage is that the driver unit 20, which (because of its electrical conductors and connections) is the most sensitve part of a loudspeaker, is safely disposed interiorly of the horn 17 where, for example, it is less susceptible to impact from falling objects. Furthermore, the means for mounting the element 22 and the driver unit 20 within the horn 17 can be made more rigid than is sometimes possible with a single threaded means which is common in the prior art. As shown in FIG. 2, two of the plurality of mounting bolts 29 are widely spaced to impart stability to the internal elements of the loudspeaker 16.

Placed across the throat 18 of the horn 17 to close the same are a pair of screens or filters 30 separated by a washer 31 and held in position by a plug 32 which threadably engages a bore in the inner element :22. The washer 31 has an aperture which is about the same size as the diameter of the adjacent portion of the throat 18, so that the washer does not restrict the passage of sound waves through the throat. The filters 30 are preferably made of porous bronze material such as is most commonly used as a fuel filter for internal combustion engines and the like. A satisfactory material is a Porex bronze filter, Grade 1 (coarse), which can be purchased from the Delco 'Moraine Division of General Motors Corporation, Dayton, Ohio. Such a filter 30 is reportedly fabricated by compressing literally thousands of small spheriods of bronze under proper heat conditions until a rigid mass results. Such a filter is represented in FIGURE 3 with slight exaggeration of the particle sizes for the purpose of clarity.

An advantageous feature of the loudspeaker 16 is that the depth of the loudspeaker as measured from its front to its rear is substantially less than its breadth as measured along a diameter. This relationship is true even when mounting tabs 33 are disregarded in measuring, as they should be since they can be located anywhere. This construction quite obviously presents a relatively thin silhouette, and is particularly advantageous whenever space is at a premium or when personnel must work near an exposed loudspeaker and they might be injured if they fell against it. The ratio of horn breadth to thorn depth in the embodiment of FIG. 2 is approximately four to one, which constitutes what is known to be an extremely efiicient design ratio.

In operation of the loudspeaker 16, the driver unit 20 receives signals through the electrical leads connected to a voice coil, and sound waves are propagated in a conventional manner by some type of vibratile element. Upon exiting the sound output aperture 21, the sound waves pass through the plurality of filters 30 and through the throat 18, after which they follow the radially expanding path between the horn walls until they exit the horn mouth 19. In accordance with convention, the driver unit 20 is said to be loaded by the horn 17; that is, the vibratile element must first drive or vibrate the air column contained in the horn, and this air column in turn will vibrate the air in the surrounding listening area. The loudspeaker 16 is thus known as a horn-loaded loudspeaker. The fact that the loudspeaker 16 is also folded does not alter its inherent horn-loaded characteristics, since within certain limits horn-loaded loudspeakers can be made in many shapes without materially affecting their efiiciency. As might be suspected, the number of filters 30 which are selected for a particular embodiment, whether it be two, three, four, etc., does have some effect on the response of the loudspeaker 16; but it has been discovered that by using a plurality of coarse filters, the diminution in response is very slight while the protection achieved against explosion is greatly increased.

As shown in FIG. 4, the curve indicated by the numeral 1 represents the response obtained with a loudspeaker 16 having a single filter 30 which is about inch thick. The curve 2 represents the same loudspeaker with two filters 30 spaced about inch apart. Curve 3 and curve 4 depict performance with three and four filters 30, respectively, each spaced about 4 inch from the adjacent filter. It will be seen that the response with four filters has almost exactly the same pattern as the response with one filter, and is only slightly lowered in intensity. By way of contrast, the curve indicated by the numeral 5 reflects the response with a single filter which is similar in thickness and shape to the coarse filter 30, but which is fabricated from much smaller particles and which is more tightly compacted. This latter filter can also be purchased from the Delco Moraine Division of General Motors Corporation, and is generally identified as a Porex bronze filter, Grade 4 (fine). The coarse filters will not allow a particle to pass therethrough which has a diameter of 0.0075 inch or larger, while the fine filter will trap all particles in excess of 0.001 inch.

As inferred earlier, the very slight diminution in loudspeaker response makes it practical to use multiple, coarse filters 30, and the protection afforded thereby makes it possible. To fully explain this, the movement of gases and gas-suspended particles must be examined as they both enter and leave the throat 18 of the horn. Further, there are at least two types of hazards to be guarded against in explosion proof loudspeakers and each must be discussed.

First, it will be assumed that the loudspeaker 16 is placed in a dusty environment, as in a flour mill, a cotton gin, etc. Further, it is assumed that the dust particles suspended in the air will tend to migrate through the filters 30 toward the driver unit 20. Those particles, however, that succeed in passing through the first filter 30 enter what may best be described as a dead space, i.e., a space which lies between two filters and interiorly of a washer 31. This dead space is somewhat analogous to a trap or a settling basin in a simple water purification system, and it promotes the settling of dust particles before they can reach the driver unit 20. When they settle, the dust particles accumulate at the bottom of the dead space and do not tend to clog up a subsequent filter; over a long period of time this is important because it keeps the filters relatively free of dust accumulations and permits relatively uninhibited passage of sound waves through the throat 18. A serial array of three filters 30 will of course be more efficient in this regard than only two such filters, and four will be more efi'lcient than three; but it has been found that two such filters provide excellent protection and will be adequate for most dusty environments.

The second type of environment in which explosionproof loudspeakers are advantageously employed is one in which the environmental gas itself is explosive, including, for example, methane, hydrogen, etc. Before the discovery described herein, it was believed that one of the fine, grade-4 filters was necessary to offer protection against this type of hazard. It is now known that in such an environment, a plurality of coarse, spaced filters 30 can offer comparable protection. Each pair of adjacent filters 30 create therebetween relatively stagnant chambers of air that inhibit the migration of environmental gas into the driver unit 20, and the trapped air between successive filters serves to dilute any potentially explosive gases that over a long period of time may eventually pass through the first filter.

Examining next the performance of the loudspeaker 16 if an explosion does somehow occur within the driver unit 20, it is found that the serially arranged filters 30 are still highly functional. Being porous, the filters 30 can begin to dissipate the excess pressure resulting from the explosion just as soon as it begins to build up, such that both the resultant rate of pressure build-up and the total pressure experienced in the driver unit 20 are reduced. The possibility of rupture of any of the components of the driver unit 20 is thereby diminished. As a result of an internal explosion, it can be assumed that hot gases will be expelled from the sound output aperture 21 of the driver unit 20 and will proceed down the throat 18 of the horn 17. In this event, the innermost filter 30 which is encountered first will somewhat impede, but will not stop, the onrushing flow of hot gases; but it will stop solid explosion products (such as likely pieces of fractured copper wire from the voice coil, etc.) The first-encountered, metallic filter 30 does, however, serve as a thermal sink for the gases; and, as they pass through the filter they begin to lose some of their heat. The captive air held between adjoining filters 30 also serves as a thermal sink, such that eventually the once-burning combustion gases are no longer hot enough to propagate a further explosion when they contact the environmental gases downstream in the air column. It will be recognized that this is the criterion for an explosion pr-oof device, and the loudspeaker 16 is therefore accurately characterized as being explosion proof.

Of further advantage is that the cover member 26 in FIG. 2 is removable from the front of the loudspeaker 16. Thus, access to the driver unit 20 is possible from the front, such that the loudspeaker can be permanently mounted without concern about later repairs in the event they should be necessary. Too, the shape of the loudspeaker itself is advantageous; thus, the thin, compact design permits flush installation in a conventional wall in a residence, apartment, etc., even if the explosion-proof features are not needed and are not incorporated. The design, while compact, does not sacrifice quality of reproduction; for the three bends permit a relatively long sound column to be included without unnecessarily enlarging the overall dimensions of the loudspeaker 16. Too, the inherent expansion achieved by causing the sound waves to expand more nearly radially than axially while they are within the loudspeaker 16 further contributes to its favorable performance-to-size ratio. The few component parts make the loudspeaker 16 attractive from a manufacturing viewpoint, and the ease with which it is assembled will, no doubt, promote numerous economies.

While only one embodiment of the invention, together with modifications thereof, have been described in detail herein and shown in the accompanying drawing, it will be evident that various further modifications are possible in the arrangement and connstruction of its components without departing from the scope of the invention.

What is claimed is:

1. A loudspeaker, comprising:

a three-bend horn having a mouth which defines the plate located in front of the driver unit and of such front of the loudspeaker and having walls which a size that when it is removed the driver unit is connect the mouth to a throat; removable from the loudspeaker in a frontal dia driver unit having a magnet structure, a voice coil, a rection.

vibratile element, and a shell, with the shell envelop- 5 References Cited ing the magnet structure, the voice coil, and the UNITED STATES PATENTS vibratile element to form an assembly which is removable from the loudspeaker as a unit, the shell 2,058,555 10/1936 Betfs et 179115-5 v ng n pening which constitutes a sound output 32%;? :32? Id iii-g aperture; and evy I explosion-proof structure which is disposed inberiorly 10 2,642,947 6/1953 Heldrlch 181-27 of the periphery of said horn, said structure sealing- 3,028,927 4/ 1962 L y et 18127 ly enclosing all but the sound output aperture of the 3,046,544 7/1962 AueT at driver unit and including a wall which is common with a segment of the horn and which has a threaded 15 STEPHEN TOMSKY Pnmary Exammer' bore that opens to the front of the loudspeaker for U S cl X R mounting the driver unit and for providing oornmunication between the sound output aperture and the 340-338 throat, said structure further including an access 

